Efficacy of Tenacity in Late Spring to Eliminate Poa Annua in Kentucky Bluegrass Fairways.

A trial was begun in late Spring 2013 on a Kentucky bluegrass fairway at the Washington State University Palouse Ridge Golf Club in Pullman, WA to determine the efficacy of Tenacity 4SC (mesotrione) combined with several herbicides to control Poa annua (post-emergence).  Treatments were: Tenacity 175 g ai/ha + Xonerate 99 g ai/ha, Tenacity 140 g ai/ha + Xonerate 49 g ai/ha, Tenacity 175 g ai/ha + Turflon 561 g ae/ha, 19660A 49 g ai/ha + Turflon 561 g ae/ha, Tenacity 175 g ai/ha + Trimmit 280 g ai/ha, and Tenacity 175 g ai/ha + Trimmit 280 g ai/ha + Turflon 561 g ae/ha.  Treatments were applied 2 May, 23 May, and 13 June 2013.  Tenacity 175 g ai/ha + Xonerate 99 g ai/ha and Tenacity 140 g ai/ha + Xonerate 49 g ai/ha resulted in the greatest reduction of P. annua, 86 and 82%, respectively, compared to the control.  However, associated with the Tenacity 175 g ai/ha + Xonerate 99 g ai/ha treatment was a high level of P. annua phytotoxicity and at times an unacceptable phytotoxicity on Kentucky bluegrass.  In addition, open depressed areas in the fairway where the P. annua had died would present undesirable playing conditions during peak summer play.  Tenacity 140 g ai/ha + Xonerate 49 g ai/ha also resulted in a high level of P. annua phytotoxicity for several weeks, but a low level of Kentucky bluegrass phytotoxicity and did not cause P. annua to quickly disappear creating open areas and depressions in the fairway.  Tenacity 140 g ai/ha + Xonerate 49 g ai/ha may be a more desirable P. annua control option to consider in terms of playability.  Complete P. annua control was not achieved with any treatment; therefore, a multi-year program may be needed to achieve this goal.